O2 scavengers are materials which can sorb oxygen. There should be understood here by sorption all the known sorption possibilities, e.g. adsorption, absorption, chemical adsorption and physical adsorption. The systems established at present according to the state of the art can be qualified here primarily according to the O2 scavenger substrate and according to the initialisation mechanism thereof. The following groups are hereby differentiated:                inorganic O2 scavengers, e.g. iron-based or sulphide-based systems        low molecular organic O2 scavengers, e.g. ascorbate-based systems        high molecular organic O2 scavengers, e.g. polyolefin-based or polyamide-based systems        
O2 scavengers are thereby initialised either by UV radiation or by moisture. This means that the O2 scavenger function is present only after exposure to UV radiation or water, i.e. air moisture.
Indicator systems in general can be subdivided into time-temperature indicator (TTI), gas/leakage indicator and freshness indicator systems.
A TTI integrates the time-temperature history of a product and hence provides direct evidence about the storage conditions thereof. The indicator effect is effected by a chemical reaction or by counter-diffusion of two colourants.
Gas-leakage indicators detect the gas concentration of O2, CO2 or H2O in the packaging space. Hence they provide direct evidence about the quality of the product. The indicator effect is caused by a chemical reaction with the reactands O2, CO2 or H2O.
Freshness indicators detect the metabolic products of microorganisms and hence provide direct evidence about the quality of the product. The indicator effect is caused by a chemical reaction of the metabolic products.
It is common to all these indicator systems that the indicator effect is reproduced by a visible colour change.
Hence there is a large number of O2 scavenger systems in the state of the art but only a decreasingly low number of gas-leakage indicator systems.
In particular for the packager and also for the seller of products (foodstuffs), the information about how the headroom atmosphere in the packaging behaves would however be important. Furthermore, with establishment of active packaging with O2 scavengers, the knowledge about the residual consumption capacity of the packaging (e.g. at the time of packaging) is of the greatest interest.